Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
1999-05-06
2001-09-25
Barefoot, Galen L. (Department: 3644)
Aeronautics and astronautics
Spacecraft
Attitude control
C244S165000, C244S171000, C244S173300
Reexamination Certificate
active
06293501
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a system for controlling the momentum of a spacecraft. More particularly, the present invention relates to a system for controlling the momentum of an asymmetric spacecraft in a sunhold attitude through a novel momentum unloading technique that does not use thrusters.
BACKGROUND ART
Many current spacecraft are required during missions to autonomously maintain a state that is power, thermal and momentum safe indefinitely in the presence of external disturbances. Indefinite power and thermal safety can be satisfied for a deployed spacecraft by placing it in a sun hold attitude. However, maintaining the spacecraft in a momentum safe state requires some momentum unloading technique.
Many attitude control strategies utilize a reaction wheel system having a limited capacity to store accumulated momentum resulting from solar pressure on a spacecraft. For symmetric spacecraft for which the solar torque is small or for spacecraft which are not required to remain in sun hold for extended periods of time, the accumulation of momentum is not a major concern and therefore, the accumulated momentum will generally not exceed the storage capacity of these reaction wheel systems.
Conversely, for asymmetric spacecraft, such as any spacecraft where instrument field of view requirements drive the spacecraft layout, solar torque can be very large relative to the capacity of the reaction wheel system to store the accumulated momentum. Further, for spacecraft which are required to operate autonomously in sun hold for long periods of time, momentum accumulation is a concern regardless of how well the spacecraft is balanced. In these cases, the accumulated momentum would quickly exceed the capacity of the system. However, it is undesirable to use thrusters to perform frequent momentum unloading operations as this increases operational risk as well as wasting valuable fuel.
Accordingly, a momentum unloading technique for an asymmetric spacecraft which is required to operate autonomously in sun hold for long periods of time without the need for the firing of thrusters would be advantageous.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a spacecraft that autonomously maintains a state that is power, thermal, and momentum safe indefinitely in the presence of external disturbances.
It is a further object of the present invention to provide a system for an asymmetric spacecraft that allows the spacecraft to operate autonomously in sun hold for long periods of time.
It is yet another object of the present invention to provide a technique for a spacecraft which uses the spacecraft body and appendages as a momentum unloading actuator.
In accordance with the objects of the present invention, a system for managing the momentum of a spacecraft is provided. The spacecraft has one or more solar panels pointed toward the sun. The spacecraft is rotating continuously or intermittently around the sunline. The spacecraft has a sun sensor for determining the location of the sun, a gyroscope for determining the roll, pitch, and yaw rates of the spacecraft, and a suite of tachometers for determining the amount of stored momentum in the reaction wheels. A controller is in communication with the sun sensor, the gyroscope, and the tachometers to determine the position, rate, and momentum of the spacecraft. The controller compares the actual position, rate, and momentum of the spacecraft to the desired position, rate and momentum of the spacecraft. If the controller determines that the position and/or rate are outside of acceptable limits, it adjusts the spacecraft attitude to maintain the sun in the sun sensor field of view in order to keep the solar panels illuminated, which is necessary for spacecraft power and thermal safely. If the controller determines that the total spacecraft momentum is outside acceptable limits, it adjusts the momentum of the spacecraft by manipulating the spacecraft orientation and/or spin rate about the sunline, and/or manipulating the angle of the solar panels.
These and other objects will become apparent from the following detailed description taken in connection with the accompanying drawings.
REFERENCES:
patent: 5349532 (1994-09-01), Tilley et al.
patent: 5794892 (1998-08-01), Salvatore
patent: 5826828 (1998-10-01), Fowell et al.
patent: 6003818 (1999-12-01), Shah et al.
patent: 6021979 (2000-02-01), Bender et al.
patent: 6076773 (2000-06-01), Salvatore
Barefoot Galen L.
Hughes Electronics Corporation
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